Trips to Mars in 39 Days

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Using traditional chemical rockets, a trip to Mars – at quickest — lasts 6 months. But a new rocket tested successfully last week could potentially cut down travel time to the Red Planet to just 39 days. The Ad Astra Rocket Company tested a plasma rocket called the VASIMR VX-200 engine, which ran at 201 kilowatts in a vacuum chamber, passing the 200-kilowatt mark for the first time. “It’s the most powerful plasma rocket in the world right now,” says Franklin Chang-Diaz, former NASA astronaut and CEO of Ad Astra. The company has also signed an agreement with NASA to test a 200-kilowatt VASIMR engine on the International Space Station in 2013.
The tests on the ISS would provide periodic boosts to the space station, which gradually drops in altitude due to atmospheric drag. ISS boosts are currently provided by spacecraft with conventional thrusters, which consume about 7.5 tons of propellant per year. By cutting this amount down to 0.3 tons, Chang-Diaz estimates that VASIMR could save NASA millions of dollars per year.

The test last week was the first time that a small-scale prototype of the company’s VASIMR (Variable Specific Impulse Magnetoplasma Rocket) rocket engine has been demonstrated at full power.

Plasma, or ion engines uses radio waves to heat gases such as hydrogen, argon, and neon, creating hot plasma. Magnetic fields force the charged plasma out the back of the engine, producing thrust in the opposite direction.

They provide much less thrust at a given moment than do chemical rockets, which means they can’t break free of the Earth’s gravity on their own. Plus, ion engines only work in a vacuum. But once in space, they can give a continuous push for years, like wind pushing a sailboat, accelerating gradually until the vehicle is moving faster than chemical rockets. They only produce a pound of thrust, but in space that’s enough to move 2 tons of cargo.

Due to the high velocity that is possible, less fuel is required than in conventional engines.

Currently, the Dawn spacecraft, on its way to the asteroids Ceres and Vesta, uses ion propulsion, which will enable it to orbit Vesta, then leave and head to Ceres. This isn’t possible with conventional rockets. Additionally, in space ion engines have a velocity ten times that of chemical rockets.

Specfic impulse and thrust graph. Credit: NASA

Rocket thrust is measured in Newtons (1 Newton is about 1/4 pound). Specific impulse is a way to describe the efficiency of rocket engines, and is measured in time (seconds). It represents the impulse (change in momentum) per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum.

Dawn’s engines have a specific impulse of 3100 seconds and a thrust of 90 mNewtons. A chemical rocket on a spacecraft might have a thrust of up to 500 Newtons, and a specific impulse of less than 1000 seconds.

The VASIMR has 4 Newtons of thrust (0.9 pounds) with a specific impulse of about 6,000 seconds.

The VASIMR has two additional important features that distinguish it from other plasma propulsion systems. It has the ability to vary the exhaust parameters (thrust and specific impulse) in order to optimally match mission requirements. This results in the lowest trip time with the highest payload for a given fuel load.

In addition, VASIMR has no physical electrodes in contact with the plasma, prolonging the engine’s lifetime and enabling a higher power density than in other designs.

To make a trip to Mars in 39 days, a 10- to 20-megawatt VASIMR engine ion engine would need to be coupled with nuclear power to dramatically shorten human transit times between planets. The shorter the trip, the less time astronauts would be exposed to space radiation, and a microgravity environment, both of which are significant hurdles for Mars missions.

VASIMR. Credit: Ad Astra

The engine would work by firing continuously during the first half of the flight to accelerate, then turning to deaccelerate the spacecraft for the second half. In addition, VASIMR could permit an abort to Earth if problems developed during the early phases of the mission, a capability not available to conventional engines.

VASIMR could also be adapted to handle the high payloads of robotic missions, and propel cargo missions with a very large payload mass fraction. Trip times and payload mass are major limitations of conventional and nuclear thermal rockets because of their inherently low specific impulse.

Chang-Diaz has been working on the development of the VASIMR concept since 1979, before founding Ad Astra in 2005 to further develop the project.

15 Responses

“the VASIMR has two additional important features that distinguish it from other plasma propulsion systems. It has the ability to vary the exhaust parameters (thrust and specific impulse) in order to optimally match mission requirements”

Point 1) Why, oh why would one choose a LOWER Specific Impulse?

Point 2) Yes, it could make it to Mars in 39 days ASSUMMING an engine 100 times larger AND a nuclear reactor! If you’re going to base your 39 day trip on non-existent technologies, why not a 20 minute trip and assume we have a rocket that can accelerate to the speed of light? (aim high I say!)

Negatives aside, I think an Ion engine may be a good fit for ISS reboosts and was an interesting article.

The VASIMR has 4 Newtons of thrust (0.9 pounds) with a specific impulse of about 6,000 seconds.

That means that 1 kg can be accelerated to a speed of 24,000 m/s, or that 24,000 kg can be accelerated to 1 m/s, or somewhere in between….another good thing is that the fuel for this engine is less massive translating into a higher payload. (note, the delta v from LEO to LMO is about 6,100 m/s, meaning that the VASIMR could take a payload of 3.93 kg to Low Mars Orbit).

More like a rocket engine, and it wasn’t tested in drift conditions (i.e. vacuum of space).

Speaking of which, Google Fast Flip aggregate made me aware today of Popular Mechanics (which I don’t read) article on a similar helicon design from MIT, which is tested in drift condition.

“NASA developed a similar engine for its Deep Space 1 Mission, launched in 1998, but the new thruster has advantages. To start, NASA employed pricey xenon gas ($13 per liter) excited into plasma by delicate electrical components, while the new design uses nitrogen (5 cents per liter) activated by a rugged radio-frequency antenna. A mag­netic field channels the plasma through a nozzle at a stunning 40 km/sec, an order of magnitude greater than the output of a chemical rocket.”

Why, oh why would one choose a LOWER Specific Impulse?

Because it is faster and making the engine more robust to throttle rf boost than throttling flow (thrust)? Or it was just another gadget in the new toy.

For your other concerns, apparently the engine scales well as seen by the test, that was the prediction going into the 200 kW demonstration.

And as regards power, that too has been demonstrated by scalable nuclear reactor designs such as goes into other vehicles. Nuclear sub reactor sizes goes up to 55 MW, and ice-breaker reactors up to 170 MW. A 90 MW design had a 1.6 m high x 1.0 m radius core @ ~ 80 kg low-enriched fuel. A 170 MW design had ~ 150 kg high-enriched fuel. [Wikipedia]

Perhaps the high-enriched fuel can be used to keep the core size down. Anyway, I don’t see anything untoward here. And as noted on a thread on NASA development on a smallish ~ 40 kW nuclear reactor for a Moon habitation, apparently nuclear reactors have been used in space before.

I think these technologies need to be pushed not necessarily for human missions but for outer-solar system missions. Mars in 29 days? Cool, but what about the fact that this sort of technology potentially makes Neptune and Uranus realistic targets for exploration again? I would literally freak out with joy if they could park a probe in orbit around Neptune and Triton – it is something I dearly hope to see in my lifetime, but technology is going to need to come a long way before that happens. This sort of tech is oxygen to that ember of hope.

“They only produce a pound of thrust, but in space that’s enough to move 2 tons of cargo.” Why the reference to 2 tons in particular??? Why not 4, or 10, or 200? According to Newton, one pound of thrust can move an arbitrarily large mass. The only difference is that the larger the mass, the less the acceleration.

@Torbjorn Larsson OM: While Russia has used over 30 fission reactors in space, some of them are so poorly shielded that they effectively blind some military and scientific satellites (mainly US satellites) with FUV, X-ray and gamma-ray detectors when they pass nearby. Hopefully most of these will be able to be safely deorbited.

I also concur with what Astrofiend said. Let’s get some robotic probes to the outer SS. Sadly, no real exploration is going to happen until we figure out how to at least approach .5C, and even more sadly, it’ll never happen in my lifetime. Still, we’re making progress.

Quantum_Flux:
You must divide exhaust velocity by the gravitational acceleration (9.81 m/sec^2) to get specific impulse in seconds. Therefore, 50,250 m/s would yield a specific impulse of 5,122 seconds.

Propulsion systems that use high-speed ejecta to slowly accelerate an object are notoriously energy inefficient; the higher the Isp, the worst the energy efficiency.

A tremendously must better system would employ an electro-dynamic pulse tether. Such tethers are only 1km long and ALL of the acceleration occurs while in earth LEO. If you accelerate an object at 1g for 621 seconds, you will have a delta V of 6,100 m/s, which is sufficient to reach Mars.
Since the tether system would do you no good at Mars, you would want to quickly decelerate it before it left LEO and allow the payload to coast all the way to Mars.
Such a tether system would require very stocky tethers, very large plasma contactors, and a very big power source (such as a flywheels, supercapacitor, or lithium ion batteries) since every metric ton of vehicle will require 18.6gigajoules (5,200 kWh) at 30MW. A recent study stated a lithium battery could produce 300 kW/kg and flywheels can achieve 1,000 kW/kg.